A toroidal-type continuously variable transmission unit adapted to be used for a transmissions of, for example, an automobile, general industrial machine, transportation equipment, etc. comprises an input disc and an output disc opposed to each other, a power roller provided between the input and output discs, a loading cam mechanism for pushing the input and output discs toward each other, etc.
The input disc can be rotated by means of a drive source such as an engine. Power that is based on the rotation of the input disc is transmitted to the output disc through the power roller. The power roller is provided for swinging motion between the input disc and the output disc. The power roller is provided with traction portions that are in rolling contact with the two discs, individually. The power roller is rotatably supported by means of a power roller bearing that functions as a thrust bearing.
The power roller bearing is provided with an end portion of the power roller that functions as an inner race, an outer race, balls provided for rolling motion between the outer race and the power roller, etc. The power roller is supported for swinging motion between the input disc and the output disc by means of a trunnion. The gear ratio of the toroidal-type continuously variable transmission unit can be changed by changing the angle of inclination of the power roller.
A power roller bearing constructed in this manner resembles a conventional thrust ball bearing that is used to support a rotating shaft on which thrust load acts except for the arrangement of the power roller. Accordingly, a study has been made to produce power roller bearings at low cost by using parts that are designed for existing thrust ball bearings.
Although a power roller bearing resembles a thrust ball bearing, its power roller functions quite differently from the inner race of the conventional thrust ball bearing. More specifically, the distribution of load that acts on the power roller itself and the behavior of the balls in contact with the outer race and the power roller considerably differ from those of the conventional thrust ball bearing, so that improvement must inevitably be made in consideration of those differences.
While the inner race of the conventional thrust ball bearing is a member that simply supports a shaft, for example, the power roller functions as a power transmitting member for transmitting torque from the input disc to the output disc. This power roller is equivalent to a transmission gear in a gear-type multistage transmission. The power roller of this type rotates at high speed under heavy forces of pressure from the input disc and the output disc, so that it generates intense heat. The heat from the power roller serves to heat the balls and the like. Thus, it is essential to use high-viscosity traction oil, which is developed for the purpose of power transmission, as lubricating oil to be supplied between the outer race and the power roller.
The traction portions of the power roller that touch the input disc and the output disc face each other at a circumferential distance of 180° on the outer peripheral edge of the power roller. The heavy forces of pressure from the input disc and the output disc intensively act on the traction, portions of the power roller as a resultant force that combines radial load and thrust load. Thus, a very high contact pressure develops in the traction portions of the power roller that touch the input disc and the output disc.
A conventional bearing is used under a contact pressure of 2 to 3 GPa (gigapascals) or less, for example. In the case of a power roller bearing that is used in a toroidal-type continuously variable-transmission unit for a vehicle, on the other hand, the contact pressure ranges from 2.5 to 3.5 GPa in a normal deceleration mode. In some cases, the contact pressure may even reach 4 GPa in a maximum deceleration mode.
Further, the heavy forces of pressure from the input disc and the output disc intensively act as a radial load in positions at a distance of 180° from each other on the traction portions of the power roller. This radial load causes the power roller to-undergo compressive deformation in the radial direction. Since this compressive deformation causes the power roller to warp, it is hardly possible to allow a plurality of balls between the power roller and the outer race uniformly to share thrust load that acts on the power roller. Thrust load that acts on these balls increases in positions at an angular distance of 90° from the positions of contact (traction portions of the power roller) between the power roller and the input and output discs. In consequence, pressures of contact between the individual balls and raceway grooves vary, so that some of the balls roll in the raceway grooves under very high contact pressures.
In order to prevent high contact pressures from lowering the life performance, it is essential specially to adjust the materials, surface hardness, and surface roughness of the traction portions of the power roller in contact with the input and output discs and the respective raceway grooves of the power roller and the outer race that are touched by the balls.
In order to lengthen the life of the power roller bearing, based on this background, the applicant hereof proposed a technique in which balls are formed of medium-carbon steel or high-carbon steel and the hardness and strength of the ball surface are adjusted by carburizing-nitriding treatment or quenching and tempering treatments (see Jpn. Pat. Appln. KOKAI Publication No. 7-208568).
The applicant hereof also proposed a technique in which an input disc, an output disc, and a power roller in contact with them are subjected to ground finish after they are subjected to carburizing treatment. Further, the applicant hereof proposed a technique in which input and output discs and a power roller are subjected to ground finish after they are subjected to carburizing-nitriding treatment so that the hardness and effective case depth of the respective surfaces of these members can be adjusted to appropriate values (2 mm to 4 mm) that stand locally high contact pressures (see Jpn. Pat. Appln. KOKAI Publication No. 7-71555).
Although the hardness, effective case depth, and surface roughness of the power roller and balls are rationalized by using traction oil for power roller bearings, use of appropriate materials for the individual members, and surface treatments, as mentioned before, the endurance of the power roller bearing can be improved only a limited amount.
In some cases, the molecular structure of the traction oil, synthetic oil, may be decomposed as the power roller is heated, in particular. In these cases, the traction coefficient worsens, and the safety factor for gloss slip also lowers. If the traction oil is degenerated, moreover, it is hard to form oil films on the respective surfaces of the power roller, balls, etc. Accordingly, there is a possibility of the traction portions of the power roller and the surfaces of the respective raceway grooves of the power roller and the outer race undergoing early flaking. These phenomena cause the life performance of the power roller bearing to lower.
Since the power roller bearing is originally designed for power transmission, it is important to minimize loss of the dynamic torque of the bearing itself, thereby improving the torque transmissibility. Despite the aforesaid improvement, however, a substantial dynamic torque loss may be caused to lower the torque transmissibility, depending on the dimensions of the balls and the respective raceway grooves of the power roller and the outer race. If the hardness and effective case depth of the respective surfaces of the power roller and the like are adjusted in the aforesaid manner, moreover, the edges of the raceway grooves or the balls sometimes may be broken early, or the respective contact surfaces of the raceway grooves and the balls may be marred, so that the life performance of the power roller bearing may be lowered.
Accordingly, the object, of the present invention is to provide a power roller bearing of a toroidal-type continuously variable transmission unit, of which the life performance can be restrained from lowering.